1. Field of the Invention
This invention relates generally to fluid dynamic bearing motors and more specifically to a fluid dynamic bearing motor optimized for radial stiffness and power consumption.
2. Description of the Background Art
FIG. 1 is an exploded perspective view illustrating a prior art disc drive 100. As shown, disc drive 100 may include, without limitation, a housing 105, a shaft 130, discs 135 and a suspension arm assembly 150. Housing 105 includes a base 110 that is attached to a cover 115. In addition, a seal 120 may be disposed in between base 110 and cover 115. Discs 135, which have surfaces 140 covered with a magnetic media configured to store information magnetically, are attached to shaft 130. During operation, suspension arm assembly 150 is configured to suspend read/write heads 145 above surfaces 140 as a spindle motor (not shown) rotates discs 135 about shaft 130 at high speed. Suspension arm assembly 150 is further configured to move read/write heads 145 radially across surfaces 140 to position read/write heads 145 above different radially spaced tracks (not shown) disposed on surfaces 140 where magnetically encoded information may be stored within the magnetic media. Once positioned, read/write heads 145 may either read magnetically encoded information from or write magnetically encoded information to the magnetic media at selected locations.
FIG. 2 is a cross-sectional view illustrating a prior art constant-gap fluid dynamic bearing motor assembly 200. Fluid dynamic bearing motors, such as constant-gap fluid dynamic bearing motor assembly (hereinafter “constant-gap FDB motor assembly”) 200, oftentimes are used in precision-oriented electronic devices to achieve better performance. For example, using a fluid dynamic bearing motor in a disc drive, such as disc drive 100 described above in conjunction with FIG. 1, results in more precise alignment between the tracks of the discs and the read/write heads. More precise alignment, in turn, allows discs to be designed with greater track densities, thereby decreasing the size of the discs and/or increasing the storage capacity of the discs.
As shown, constant-gap FDB motor assembly 200 includes, without limitation, a rotational assembly 201 and a sleeve 206. Rotational assembly 201 generally comprises the rotating elements of constant-gap FDB motor assembly 200. In the configuration shown, rotational assembly 201 includes, without limitation, a hub 202, a shaft 204 and discs 208.
Shaft 204 is attached to hub 202 and provides axial support for constant-gap FDB motor assembly 200. Hub 202 is configured to rotate about a rotational axis 205. Specifically, a magnet assembly (not shown) is attached to hub 302, and the electromagnetic interaction between that magnet assembly and a stator assembly (also not shown) causes hub 202 to rotate. As shaft 204 is attached to hub 202, shaft 204 rotates about rotational axis 205 as well. Discs 208 are coupled to the outside of hub 202 and thus also rotate about rotational axis 205 with hub 202. Sleeve 206 is configured to remain stationary.
Constant-gap FDB motor assembly 200 also includes fluid dynamic journal bearings 210 and 214 and fluid dynamic thrust bearings 218 and 220. Fluid dynamic journal bearings 210 and 214 are disposed between sleeve 206 and shaft 204. Fluid dynamic journal bearing 210 is configured with a bearing length 211 and a bearing gap 212, and fluid dynamic journal bearing 214 is configured with a bearing length 215 and a bearing gap 216. As configured, bearing gap 212 and bearing gap 216 are the same size. Fluid dynamic thrust bearings 218 and 220 are disposed between sleeve 206 and the facing surfaces of a flange 219 of shaft 204. Each of fluid dynamic journal bearings 210 and 214 and fluid dynamic thrust bearings 218 and 220 includes at least one bearing surface having a grooved bearing pattern. As is commonly known in the art, these grooved bearing patterns are configured to generate a localized high pressure region within the bearing fluid that supports the relative rotation of the surfaces of fluid dynamic bearing.
As FIG. 2 also shows, a center of gravity 222 of rotational assembly 201 is disposed between fluid dynamic journal bearings 210 and 214. Ideally, center of gravity 222 should be disposed equidistant from fluid dynamic journal bearings 210 and 214 along rotational axis 205 such that fluid dynamic journal bearings 210 and 214 equally support the radial load generated by the rotation of rotational assembly 201 about rotational axis 205. If, however, center of gravity 222 is disposed closer to one of fluid dynamic journal bearings 210 or 214, then that fluid dynamic journal bearing supports a greater radial load than the other fluid dynamic journal bearing. Such a load imbalance, if left unchecked, typically increases the operational vibration and non-repetitive run-out of constant-gap FDB motor assembly 200, thereby decreasing performance.
To compensate for such a load imbalance, the radial stiffness of the fluid dynamic journal bearing disposed closest to center of gravity 222 oftentimes is increased. Increasing the stiffness of a fluid dynamic journal bearing typically is accomplished by increasing the length of that fluid dynamic journal bearing. For example, suppose that center of gravity 222 is disposed along rotational axis 205 closer to fluid dynamic journal bearing 210 than to fluid dynamic journal bearing 214 such that fluid dynamic journal bearing 210 supports a greater radial load than fluid dynamic journal bearing 214. To compensate for the load imbalance, the radial stiffness of fluid dynamic journal bearing 210 is increased relative to that of fluid dynamic journal bearing 214 by increasing bearing length 211 relative to bearing length 215.
One drawback of this approach to tuning radial stiffness is that the radial stiffness and the power consumption of a fluid dynamic journal bearing are both equally sensitive to a change in bearing length. Thus, increasing the bearing length of a fluid dynamic journal bearing to increase radial stiffness results in a proportional increase in power consumption.
Another drawback of this tuning approach is that, although fluid dynamic journal bearing 214 ends up having less radial stiffness than fluid dynamic journal bearing 210 because bearing length 215 is shorter than bearing length 211, the radial stiffness of fluid dynamic journal bearing 214 nonetheless is oftentimes too great in relation to the smaller radial load that fluid dynamic journal bearing 214 supports. Such an over-design needlessly increases the power consumption of fluid dynamic journal bearing 214 and, thus, constant-gap FDB motor assembly 200.